Python lexical structure

Computer languages, like human languages, have a lexical structure. A source code of a Python program consists of tokens. Tokens are atomic code elements. In Python language, we have comments, variables, literals, operators, delimiters, and keywords.

Comments

Comments are used by humans to clarify the source code. All comments in Python language follow the # character.

comments.py
#!/usr/bin/python3

# comments.py
# Author Jan Bodnar
# ZetCode 2017

def main():
    print("Comments example")

main()

Everything that follows the # character is ignored by the Python interpreter.

Variables

A variable is an identifier which holds a value. In programming we say that we assign a value to a variable. Technically speaking, a variable is a reference to a computer memory, where the value is stored. In Python language, a variable can hold a string, a number, or various objects like a function or a class. Variables can be assigned different values over time.

Variables in Python can be created from alphanumeric characters and the underscore _ character. A variable cannot begin with a number. This way the Python interpreter can easier distinguish between a number and a variable.

Value
value2
company_name

These are valid identifiers.

12Val
exx$
first-name

These are examples of invalid identifiers.

The variables are case sensitive. This means that Price, price, and PRICE are three different identifiers.

case.py
#!/usr/bin/python3

number = 10
Number = 11
NUMBER = 12

print(number, Number, NUMBER)

In our script, we assign three numeric values to three identifiers. While it is possible to define three variables differing in their case, it is not considered a good programming practise.

$ ./case.py
10 11 12

This is the output of the script.

A literal

A literal is any notation for representing a value in a Python source code. Technically, a literal is assigned a value at compile time, while a variable is assigned at runtime.

age = 29
nationality = "Hungarian"

Here we assign two literals to variables; number 29 and string "Hungarian" are literals.

literals.py
#!/usr/bin/python3

# literals.py

name1 = "Jane"
age1 = 12

name2 = "Rose"
age2 = 16

"Patrick"
34

"Luke"
23

print(name1, age1)
print(name2, age2)

If we do not assign a literal to a variable, there is no way how we can work with it. It is dropped.

$ ./literals.py 
Jane 12
Rose 16

This is the output of the literals.py script.

Operators

An operator is a symbol used to perform an action on some value.

+    -    ~    *    **    /    //
%    <<    >>    &    |    ^
and    or    not    in    not in 
is    is not    <    >    !=    <>
==    <=    >=

This is a list of operators available in Python language. We will talk about operators later in the tutorial.

Indentation

Indentation is used to delimit blocks in Python. Where other programming languages use curly brackets or keywords such as begin, end, Python uses white space. An increase in indentation comes after certain statements; a decrease in indentation signifies the end of the current block. The Python style guide recommends using four spaces per indentation level.

if age > 18:
    print("adult person")

for i in range(5):
    print(i)

After the if keyword a code block is expected. A new statement is started on a new line, indented with four space characters. The indentation for the following for keyword is decreased back to the initial one. The for keyword starts a new code block, where its statement(s) are indented.

Delimiters

A delimiter is a sequence of one or more characters used to specify the boundary between separate, independent regions in plain text or other data stream.

(       )       [       ]       {       }      
,       :       .       `       =       ;
+=      -=      *=      /=      //=     %=
<=      |=      ^=      >>=     <<=     **=
'       "       \       @

Delimiters are used in various area of the Python language. They are used to build expressions, string literals, tuples, dictionaries, or lists.

Keywords

A keyword is a reserved word in the Python programming language. Keywords are used to perform a specific task in a computer program. For example, print a value to the console, do repetitive tasks or perform logical operations. A programmer cannot use a keyword as an ordinary variable.

and       del       global      not      with   
as        elif      if          or       yield
assert    else      import      pass     False
break     except    in          raise    None 
class     finally   is          return   True     
continue  for       lambda      try     
def       from      nonlocal    while     

This is a list of Python keywords. We have dedicated a special chapter to keywords in this tutorial.

This was the Python lexical structure.